1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus configured to supply toner from a toner bottle to a developing device.
2. Description of the Related Art
Some of image forming apparatuses including a developing device for developing toner to form an image, such as electrophotographic copiers and electrophotographic printers, are configured to have a toner bottle inside thereof to supply toner from the toner bottle to the developing device. The amount of toner in the developing device is monitored by a toner sensor, and at a certain level or lower, toner is automatically supplied from the toner bottle.
When such an image forming apparatus is not operated for a long time, toner in the developing device settles and the bulk density of toner changes. The waveform detected by the toner sensor changes with the changing bulk density of toner. As a result, the amount of toner in the developing device cannot be detected accurately by the toner sensor. If toner is supplied to the developing device based on such sensor output, an inappropriate amount of toner may be supplied. For example, because of the changing bulk density of toner, the amount detected by the toner sensor may be smaller than the actual amount of toner in the developing device. If toner is supplied to the developing device based on such sensor output, the toner in the developing device becomes too much, possibly causing an overflow.
In order to solve the aforementioned problem caused by the changing bulk density of toner, for example, Japanese Laid-Open Patent Publication No. 2003-215910 discloses a technique of agitating toner in a buffer at “a prescribed timing” (developing agitation). This technique aims to prevent a change of bulk density by agitating toner in the developing device at a prescribed timing and thereby to prevent increase of loads on the developing sleeve or the agitating screw.
According to the technique disclosed in the document above, whether developing agitation before use is required or not this time is determined by comparing the toner sensor value during the last use with the toner sensor value during the current use. This technique, however, requires developing agitation for measurement in order to obtain the more accurate toner sensor value during the last use. Accordingly, unnecessary developing agitation is performed. Furthermore, in this technique, if it is determined that developing agitation is necessary as a result of the comparison of toner sensor values, developing agitation is performed without consideration of other circumstances, and therefore developing agitation that is not always necessary depending on other circumstances may be performed.
Toner is deteriorated with developing agitation. Therefore, the developing agitation performed as a result of the determination, as in the technique disclosed in the document above, may lead to deterioration of toner. To prevent deterioration of toner, it is effective to reduce the frequency of developing agitation. In other words, it is effective to properly set the above-noted “prescribed timing”.
In order to reduce the frequency of developing agitation, for example, a technique may be added in which a notice is given to indicate the time during which the image forming apparatus is not operated. In this case, however, a clock function is necessary for counting the time during which the image forming apparatus is not operated. Thus, this technique cannot be applied to an image forming apparatus that does not have a clock function.